The present invention is directed to a method for preparing liquid fuel in a hydrocarbon synthesis process, and more specifically for preparing a stabilized mixed fuel from a carbon source at a remote site, and tailoring one or more finished fuel products from the mixed fuel in order to meet local fuel requirements at a market site.
Many of the huge natural gas reserves which are targeted for hydrocarbon fuel synthesis processes, such as a Fischer-Tropsch process, methanol synthesis and the like are generally remote from major fuel markets. Consequently, most or all of the products from the synthesis process are exported from the remote site, generally to multiple markets, each with potentially different fuel needs and requirements.
In the conventional synthesis process, a natural gas, coal or heavy oil is generally converted to liquid hydrocarbons at a site adjacent to the natural resource. In the Fischer-Tropsch process, a carbon-based resource is converted to syngas (predominantly CO and H2) and the syngas converted to a primarily paraffinic hydrocarbon product. In preparing a fuel, the paraffinic hydrocarbons in the conventional process are hydroprocessed to remove at least some of the oxygenates and olefins in the product, to reduce the molecular weight of the product, and to lower the cloud point and/or the pour point of the product. A final distillation step in the conventional process provides the finished fuel and lube product for export to the various markets.
However, the conventional method has several disadvantages. For one, the conventional processes are complex and expensive, with multiple processing steps conducted at the remote site. Developing the site and transporting equipment to the site is generally more costly that using existing processes at a more developed market site. Furthermore, much of the product from a remote site process is exported, generally to more than one market site. Each of these market sites have potentially different fuel requirements and needs. Finished fuel product prepared at the remote site must be tailored to meet the specific requirement of each market.
Methods for transporting Fischer-Tropsch derived syncrude from a remote site to a commercial refinery are known in the art (See, for example, U.S. Pat. Nos. 5,968,991; 5,945,459; 5,856,261; 5,856,260 and 5,863,856). One approach has been to isolate a C20-36 syncrude and ship this composition as a solid. A limitation of this approach is that it is difficult and expensive to transport solids, because it requires expensive forming, loading and unloading facilities.
Other approaches have focused on transporting syncrude, or a syncrude which has been partially refined to convert some of the linear hydrocarbons into isoparaffins and thus generate a syncrude which is liquid at near ambient temperature. One approach to transporting syncrude in the liquid state involves partially dewaxing the syncrude to form a pumpable liquid (See, for example, U.S. Pat. No. 5,292,989). However, this dewaxing may require the construction of facilities which are expensive and difficult to operate in remote locations.
Another approach involves transporting the syncrude as a molten wax. This transportation method does not require the forming, loading and unloading facilities needed to transport solids, or the dewaxing facilities needed to convert the syncrude into a product that is liquid at room temperature. However, Fischer-Tropsch products include a sufficient quantity of volatile hydrocarbons to cause the products to exceed the vapor pressure specifications if the syncrude were shipped at a temperature at which the syncrude is molten.
What is needed is a process for preparing a finished fuel from a remote hydrocarbon synthesis process, while reducing the processing complexity of the process at the remote site. What is also needed is a more effective method for tailoring the final product from the synthesis process for each individual market.
The present invention relates to a Fischer-Tropsch synthesis process, and to an integrated process for preparing a stabilized product mixture, in a Fischer-Tropsch synthesis process, for export to a market location. In the process, a carbonaceous source which is recovered from a remote site is converted through a series of steps into a stabilized product mixture at or near the remote site. At least a portion of stabilized product mixture is then exported to a market location, for a final separation step to produce at least one finished fuel product. More specifically, in the present process, a carbonaceous source, such as natural gas, coal or heavy oil, which is recovered as a resource at a remote site, is converted to a syngas comprising predominantly H2 and CO. The syngas is further converted to synthetic hydrocarbons in a hydrocarbon synthesis process, and the synthetic hydrocarbon product so produced are converted to a stabilized product mixture for export to a market location. A Fischer-Tropsch synthesis process is the preferred process for preparing the synthetic hydrocarbons.
In the preparation of the stabilized product mixture, the synthetic hydrocarbon product is upgraded via hydroprocessing, at conditions selected to yield a stabilized product mixture which comprises fuel and/or lubricating oil base stock range products. At least one of the products present in the stabilized product mixture has the properties of a finished fuel product, and can be recovered as such by an additional distillation step.
In one embodiment, the invention provides a method for preparing finished products from a Fischer-Tropsch synthesis process, the method comprising:
(a) reacting a synthesis gas comprising H2 and CO to form at least one Fischer-Tropsch effluent product;
(b) reacting at least a portion of the Fischer-Tropsch effluent product at hydroprocessing conditions to form a hydroprocessed effluent;
(c) separating at least a portion of the hydroprocessed effluent into at least a C4xe2x88x92 fraction and a stabilized product mixture;
(d) transporting at least a portion of the stabilized product mixture to a market site; and
(e) separating at least a portion of the stabilized product mixture at the market site into at least one finished fuel product.
In a separate embodiment, the invention provides a method for preparing finished products from a Fischer-Tropsch synthesis process, the method comprising:
a) receiving a stabilized product mixture recovered from a Fischer-Tropsch synthesis process, which stabilized product mixture is prepared by the process comprising:
i) reacting a synthesis gas comprising H2 and CO to form at least one Fischer-Tropsch effluent product;
ii) reacting at least a portion of the Fischer-Tropsch effluent product at hydroprocessing conditions to form a hydroprocessed effluent;
iii) separating at least a portion of the hydroprocessed effluent into at least a C4xe2x88x92 fraction and a stabilized product mixture; and
iv) transporting at least a portion of the stabilized product mixture to a market site;
b) separating at least a portion of the stabilized product mixture without additional hydroprocessing into at least one finished fuel fraction at the market site;
wherein the stabilized product mixture is prepared at a remote site relative to the market site.
A preferred stabilized product mixture comprises:
(a) greater than 80 wt % paraffins,
(b) less than 200 ppm oxygen as oxygenates,
(c) less than 50 ppm sulfur,
(d) less than 50 ppm nitrogen, and
(e) less than 5% (v/v) olefins.
For mixtures of this type, most or all of the stabilized product mixture is recovered from a step of hydroprocessing.
Among other factors, the present invention includes Fischer-Tropsch synthesis, upgrading the synthesis product (preferably by one or more of hydrotreating, isomerization and hydrocracking), and stabilizing the resultant full boiling range liquid product. The process further includes separating the full boiling range liquid product to final finished products meeting specification requirements. In the process of the invention, all the major processing steps are conducted at a remote site except for final fractionation, which is conducted at a market site. This invention significantly reduces remote site operating complexity by moving the final distillation step from the remote site to the market site, with equipment suitable for distillation and processing in order to make the finished fuel product. Since final product separation is carried out at the market site, final product separation can be tailored to the particular market to which the stabilized product mixture is exported, rather than being anticipated at the remote site.